11. Field of the Invention
The present invention relates to an active matrix substrate comprising an array of thin film transistors formed on an insulating substrate, which constitutes an active matrix display device in combination with a liquid crystal as a display medium and the like.
2. Description of the Prior Art
An active matrix liquid crystal display device employing a "chip on glass" (COG) method is known, in which driver ICs supplying drive signals to source buses and gate buses for a thin film transistor (TFT) array are directly mounted in the form of a chip on an insulating substrate on which the TFT array together with the source buses and the gate buses is formed.
FIG. 7 shows the above type conventional active matrix liquid crystal display device, which comprises a TFT array 2 formed with TFTs arranged in a matrix on a glass substrate 1. Source buses 20 and gate buses 21 running through the area of the TFT array 2 so as to be connected to each TFT are connected to a source bus driver IC 3 and a gate bus driver IC 4, respectively. These driver ICs 3 and 4 are also mounted on the glass substrate 1 in the form of a chip. A counter glass substrate 5 is placed over the whole area of the TFT array 2 with a liquid crystal portion 7 interposed therebetween.
FIG. 8 shows the structure of the source bus driver IC 3 mounted on the glass substrate 1 and connected with the source bus 20. The source bus 20 is covered with an insulating film 6 with an exception of an uncovered COG connection 30 where the source bus 20 is connected to the source bus driver IC 3. The COG connection 30 is positioned underneath the source bus driver IC 3, thereby protecting the uncovered metal of the source bus 20 from being exposed to the outside when the source bus driver IC 3 is mounted on the substrate. The gate bus driver IC 4 mounted on the glass substrate 1 and connected with the gate bus 21 has the same structure as the source bus driver IC 3 as described above.
Generally, in an active matrix liquid crystal display device, the quality of display may be degraded because of an electrical factor. In such a case, the waveform of a signal input to the source bus 20 or the gate bus 21 can be monitored and inspected during the drive of the liquid crystal panel having an area corresponding to the area occupied by the TFT array 2, so as to analyze the cause.
The monitoring of the signal waveform is normally performed by putting a probe in contact with the metal of the bus. However, in the above conventional active matrix liquid crystal display device, this monitoring during the drive of the liquid crystal panel is not possible, because, as described above, no part of the metal is exposed to the outside for providing an access for probing. The monitoring may be possible if the insulating film 6 covering the bus is partly removed to form an exposed metal portion on the bus. However, it is difficult to remove only the insulating film 6 from the metal, and further, if the insulating film 6 is successfully removed, it is also difficult to put the probe in contact with the bus which is fine and thin (width: 10 .mu.m, thickness: 400 nm) without causing troubles such as breakage and cutting.
The objective of the present invention is to provide an active matrix substrate in which the waveform of signals input to the buses can be easily monitored by visual probing without troubles such as breakage and cutting.